1. Field of Invention
The present invention relates to a pyrometallurgical smelting method of copper, and more particularly to an improvement of a method for charging the carbonaceous material into a flash smelting furnace which is utilized for the pyrometallurgical smelting of copper.
In the smelting operation of copper, a portion of Fe in the charged materials undergoes over-oxidation to form magnetite (Fe.sub.3 O.sub.4) in the slag. This Fe.sub.3 O.sub.4 deposits on the bottom or sidewall of the flash smelting furnace and acts as the protecting layer on the refractories of the furnace but, on the other hand, decreases the furnace's inner capacity. When the amount of Fe.sub.3 O.sub.4 so formed becomes such that excessive growth of the coating is incurred, the coating may finally clog a tap hole for the slag and matte, so that the tapping operation is made difficult. In addition, a semi-molten solid layer, i.e., the so-called intermediate layer, is formed between the slag and matte layers in the furnace, thereby impeding separation of the slag and matte layers from one another. Furthermore, since the viscosity of the slag is increased by Fe.sub.3 O.sub.4, the amount of copper suspended in the slag, and hence the waste amount of copper, increases. Various troubles as described above are incurred when magnetite is formed in a large amount. It is therefore important in the light of achieving effective and stable smelting operation of copper to suppress the amount of Fe.sub.3 O.sub.4 to a very low level.
2. Description of Related Arts
It is a known process in the flash smelting of copper to blow powder coke with or without finely particulated coal together with copper concentrate and heavy oil into a flash smelting furnace so as to decrease the copper loss in the tapped slag and also to minimize fuel consumption (Japanese Unexamined Patent Publication No. 58-221,241). According to descriptions of this publication, since the metallurgical reactions suddenly occur in the oxidizing atmosphere of the flash-smelting furnace, a large amount of Fe.sub.3 O.sub.4, which is a peroxide of iron, is formed and contained in the slag. The unburnt powder coke, which covers the slag, is therefore, caused to react with the magnetite and reduces it. The copper loss in the slag is decreased along with reduction of magnetite.
In addition, according to Japanese Unexamined Patent Publication No. 58-221,241 mentioned above, there are descriptions about the following preferred methods: the powder coke is added in the reaction shaft of a flash smelting furnace in such a manner that the entire surface of melt in the settler is uniformly covered with the unburnt powder coke; regarding the grain size of coke, since the degree of reduction of magnetite decreases when the grain size is ultra-fine, grain size is preferably from 16 mesh (1 mm) to 325 mesh (44 .mu.m); and the carbonaceous material should have a high content of volatile matters.
Saganoseki Smelter, which belongs to the present Assignee, used, in a flash-smelting furnace, powder coke having the following distribution of grain sizes and attained from 2 to 4% of magnetite level in the slag. Also, consideration was given to the fact that the unburnt coke, which floats on the slag surface, reduces a portion of the magnetite ("Non-ferrous Smelting and Energy Saving" (1985) edited by Research Committee Concerning Non-ferrous Smelting Techniques and Energy. This Committee is organized under Japan Society for Mining and carried out research into the use of powder coke in a flash smelting furnace.
TABLE 1 ______________________________________ Kind of powder coke A B C ______________________________________ Distribu- over 10 mm 0 0 0 tion of 5-10 mm 6 6 5 grains 3-5 mm 4 5 9 1-3 mm 16 25 21 0.15-1 mm 42 50 55 under 0.15 mm 32 14 10 total 100 100 100 Components Free carbon 85 85 85 (%) Volatile 1 1 2 matters Ash and 14 14 13 others Heat value 6,800 6,800 7,000 (kcal/kg) ______________________________________
As described hereinabove, the process that is widely used at present in the copper smelting operation with the use of a flash-smelting furnace is to charge powder coke, finely particulated coal, finely particulated coke and the like into a reaction shaft for the purpose of reducing Fe.sub.3 O.sub.4 and preventing troubles arising from the excessive formation of Fe.sub.3 O.sub.4 described above. More specifically, although heavy oil, powder coke, finely particulated coal and the like have heretofore been charged into the reaction shaft of a flash-smelting furnace and burnt as a measure for heat compensation, a portion of the powder coke and finely particulated powder is not burnt in a reaction shaft and enters the melt formed at the bottom of the reaction shaft. Fe.sub.3 O.sub.4 in the slag is then reduced by the unburnt coke. In other words, the powder coke and the like are added in the reaction shaft as a measure for heat compensation and also as an effective measure for reducing Fe.sub.3 O.sub.4.
In a pyrometallurgical smelting method of copper with the addition of carbonaceous material, when the carbonaceous material is inadequately charged so as to result in the excessive reduction of magnetite, the coating on the furnace is diminished and the refractories are subjected to strong erosion. This causes such various drawbacks as: leak of melt from the furnace, formation of a metallic layer in the furnace, intrusion of metal into the masonry joints between bricks in the furnace bottom and hence causing upheaving of the bricks; partition of impurities into the metallic layer thereby lowering their distribution into the slag layer; and, transportation of the unburnt carbonaceous material upward to the waste-heat boiler where it is burnt, which seriously impedes the boiler operation.
As is explained in Japanese Unexamined Patent Publication No. 58-221,241 and the technical report by the smelter of the assignee, when the surface of the slag bath is covered by the unburnt powder coke, the amount of which is excessive from the view point of the intended purpose, it stagnates on the slag bath and drastically lowers the equilibrium partial pressure of oxygen. The thus formed highly reducing atmosphere in the furnace incurs in most cases such troubles as: disappearance of the coating on the furnace refractories and hence causing their erosion; upheaving of bricks due to intrusion of metal into bottom bricks; and decrease in the degree of impurity removal into the slag phase. The unburnt carbonaceous material generated in a large amount is transported together with gas into the waste-gas boiler and is later burnt there.